The present invention relates generally to an ionization chamber device for detecting the presence of ionizing radiation and, in particular, the presence of beta radiation particles and, if beta radiation particles are present, providing a means whereby the amount of radiation dose due to beta radiation particles present may be accurately calculated and displayed. If desired, the device may also serve to detect the presence and calculate and display the amount of radiation dose due to other types of radiation, for example gamma radiation particles.
Virtually ever since the discovery and subsequent utilization of atomic energy it has been known that the presence of various types of radiation particles, such as, for example, alpha, beta and/or gamma particles, is closely associated with the differing sources of atomic energy. Since exposure of individuals to these particles can be quite detrimental to their health and general welfare, it is highly desirable, if not mandatory, that the amount of radiation present in any location be readily and accurately ascertained so that areas of high exposure can either be decontaminated, shielded, or avoided. Ionizing radiation is usually measured in rads per hour for a given unit volume. A "rad" is the unit of absorbed dose in the field of radiation dosimetry. One rad equals the absorption of energy in any medium of 100 ergs per gram.
In response to this long-standing and continuing need for an arrangement for quickly and accurately determining whether a given location has been adversely contaminated with material that emits one or more types of radiation, several radiation particle detectors have been developed over the years. At the heart of these detectors is a chamber located within a housing and generally defined by electrically conductive walls which terminate in an edge that defines an opening into the chamber. Since some types of radiation particles, for example beta radiation particles, are incapable of penetrating the customary materials from which the housing of the detector is usually manufactured, the opening is covered by an electrically conductive window material that is pervious to the type of radiation which the detector is designed to locate so as to permit passage of the radiation particles through the window and opening and into the chamber. Conventional electrically conductive windows are formed by applying a light coating of a conductive material, such as aluminum, onto the window. Covering of the opening by the window material results in a quantity of gaseous molecules being entrapped within the chamber. In most instances the gaseous molecules that are trapped within the chamber comprise normal atmosphere, that is the mixture of various gaseous materials which is usually included within ambient air. However, some of the past radiation detectors have included a chamber which is air-tight and which has been filled with a specific gaseous material, for example, argon. An important aspect of the molecules of the gaseous material which are trapped within the chamber is that they should ionize upon their impact with a particle of the type of radiation which the detector is designed to locate. An electrode, which is physically separated and thus electrically insulated from the electrically conductive walls and window of the chamber, is disposed within the chamber and is electrically connected, by way of a conventional electrical circuit, to a source of electricity located outside of the chamber which, for ease of handling, is preferably a portable source of electricity such as a battery. The source of electricity, e.g. battery, is, in turn, connected, by way of a conventional electrical circuit, to the electrically conductive walls and windows of the chamber so that an electrical potential exists within the chamber between the electrode and the electrically conductive walls of the chamber and between the electrode and the electrically conductive window. A conventional on-off switch has usually been included within the electrical circuit so that the battery and thus the electrical potential existing within the chamber may be selectively activated. Additionally, a conventional meter has also usually been included at some point within the electrical circuit so that any electrical current flowing through the circuit may be detected and measured. The meter is usually powered by being connected, by a second, separate conventional electrical circuit, to its own source of electricity, e.g. a second battery. The meter is usually calibrated to display the amount, in rads per hour, of radiation present.
In the radiation detection or location mode, the on-off switch will be in the "on" mode and, for the reasons discussed above, an electrical potential will exist within the chamber. Accordingly, if a source of radiation particles is present and the radiation pervious window of the detector is oriented so that the radiation particles may pass through the window and the opening into the chamber, the radiation particles, upon entry into the chamber, will impact with the ionizable gaseous molecules located within the chamber and the molecules will be transformed into ions. As a result of the electrical potential existing within the chamber, the ions, depending upon whether they are positively or negatively charged, will flow to either the electrode or the electrically conductive walls/window of the chamber and an electrical current will flow within the electrical circuit connecting the electrode to the walls/window of the chamber. As is well known, the number of ions formed within the chamber and thus the magnitude of the electric current flowing through the circuit between the electrode and the walls of the chamber is directly related to and will vary with both the number of radiation particles entering the chamber and the number of molecules which are located within the chamber and which are capable of being ionized by the radiation particles. As is also well known, the number of molecules capable of being ionized within the chamber is dependent upon the density of the gaseous material trapped within the chamber and the size of the radiation accessible, e.g. radiation sensitive, volume of the chamber, with the number of molecules for a known gaseous material being readily calculable for a known radiation sensitive volume. The radiation sensitive volume of the chamber is less then the total volume of the chamber since the volume which the electrode and any other object within the chamber occupies must be discounted from the total chamber volume. Additionally, where the detector is to be utilized to detect radiation particles which not only cannot penetrate the housing of the detector but also cannot penetrate the electrode located within the chamber, for example beta radiation particles, any area of the chamber which is inaccesible to the beta radiation particles as a result of that area being shielded from the particles by the electrode must also be discounted from the total volume of the chamber. If the detector is to be used to detect the presence of particles which can penetrate the housing, for example gamma radiation particles, a moveable plate or cover which is impervious to beta radiation particles is usually placed over the beta radiation pervious window so that the ionization chamber is completely shielded from beta radiation particles as a result of the chamber being shieldingly surrounded by the combination of the housing and the cover. The resultant electrical current flowing through the circuit between the electrically conductive walls/window and the electrode will be generated solely by radiation particles which are capable of penetrating the housing/cover combination, e.g. gamma radiation particles. Accordingly, the meter of the detector will display the amount of gamma radiation dose rate present, preferably in rads/hour. If beta radiation particles are also present and the cover is removed from over the beta radiation pervious window the magnitude of the electrical current will increase due to increased ionization of the gaseous molecules within the chamber as a result of entry by the beta radiation dose rate into the chamber being permitted. The amount of beta radiation dose rate present is represented by the amount of the increase in the magnitude of the electric current and may be obtained by subtracting the gamma radiation reading from the total radiation reading. A problem with this operation is that the volume of the ionization chamber which is sensitive to beta radiation particles will be different from the volume of the ionization chamber which is sensitive to gamma radiation particles. This problem which will be more thoroughly discussed hereinafter has been approached by prior detectors by including, within the detectors, logic circuitry instructing the detector to disregard any area of the ionization chamber which is not accessible to a particular type of radiation particle when calculating the number of molecules available for ionization.
Conventionally, past detectors have been calibrated by orienting the radiation pervious window and thus the ionization chamber toward a radiation source of known intensity and observing the amount of current flow registered by the meter. Since the amount of radiation dose rate present is related to the amount of current flow by well known conventional equations, the entire range of the detector may, accordingly, be readily calibrated by known methods and the detector may thereafter be utilized by those in the field for its intended purpose.
Unfortunately, past radiation detectors have been subject to persistent problems involving both their inaccuracy and imprecision. These problems have been quite vexatious to those who work in the area in view of the fact that, for the reasons stated above, it is highly critical that they be correctly apprised of the degree of radiation to which they are exposing themselves at all times. Even more vexatious to those in the area was the fact that it appeared that the source of both the inaccuracy and imprecision of the prior detectors was inherent in the chamber/electrode design, itself. Accordingly, it is clear that a need existed for a beta radiation particle detector having improved accuracy and precision and which is also capable of readily separately and individually measuring multiple types of radiation particles.